Outside of battle put the item you want to duplicate in the top left or top right corner of the item menu. Then go into a random battle go to the item menu in battle. While there go to the item that is in the corner and hit up and A at the same time. If it works you should see the equipment screen for about a 1/2 second, then it will select who you want to use it on. Hit the B button and then the item should be duplicated (only items you can use in battle can be duplicated weapons and armour can't).
Added by: petris133 Dec 7th 2007, ID#5677